Rotfiends can be found in Velen.
They are weak against necrophage oil.
Rotfiends resemble decomposing human bodies that have been stripped of their skin. Their presence is given away by the overwhelming stench of the rot which gives them their name. Devourers are a particularly dangerous kind of rotfiend marked by an insatiable appetite for human flesh.
You're likely in for a shock the first time that you come across a rotfiend. They sport a similar set of attacks to the common drowner, but the ace up a rotfiend's sleeve is that it'll explode in a poisonous cloud when its health reaches a certain point.
This point is usually when a rotfiend's health bar reaches the one quarter mark. It'll stand still, quiver, and then start to expand. Needless to say, it's in your best interest to dive away from the ensuing blast. Other than that, rotfiends are relatively easy to bring down. Igni can be good for keeping them at bay, or it can even be used to knock just enough health away that they detonate from a safe distance.